Our
profiles were interpolated to give
(hereafter just m) at the radii which the given literature source had used.
The interpolation was done by fitting a quadratic function to the profile.
In the cases of JFK92 and JFK95a, for which we had access
to their
m(r)
profiles, the opposite was done:
their magnitudes were interpolated to two of our apertures:
6.26'' and 10.08''.

Where we had observed a given galaxy more than once, a mean of our values
was calculated before calculating the difference with the given
literature source.

Note, that throughout this thesis, aperture refers to the
radius of the aperture, not the diameter.

Weedman (1976):
Photoelectric photometry in Johnson U.
Magnitudes probably not cleaned for foreground stars
and other galaxies, but the used aperture is also quite small.
3 galaxies in common: R234, R256, and R269.
1 aperture: 8.25''.

The result of the external comparisons is given in
Table and Figure .

Table:
External Comparison of Aperture Magnitudes

Gunn r

Source

N

mean

rms

JFK92, 6''

3

1

3

0.002

JFK92, 10''

3

1

3

0.007

JFK95a, 6''

14

1

14

0.018

JFK95a, 10''

14

1

14

0.015

Poulain & Nieto (1994)

2

5

10

0.045

Poulain & Nieto (1994)

2

4-5

9

0.031

Johnson B

Source

N

mean

rms

Burstein et al. (1987)

4

3

12

0.071

Burstein et al. (1987)

4

2-3

11

0.052

JFK92, 6''

3

1

3

0.003

JFK92, 10''

3

1

3

0.015

Poulain & Nieto (1994)

2

5

10

0.045

Poulain & Nieto (1994)

2

4-5

9

0.027

Johnson U

Source

N

mean

rms

Poulain & Nieto (1994)

1

4

4

0.09

Poulain & Nieto (1994)

1

3

3

0.02

Sandage (1975)

2

1

2

0.04

van den Bergh (1977)

3

1-4

7

0.18

Weedman (1976)

3

1

3

0.01

Notes:
is the number of galaxies in common with the given source in the
given filter.
is the number of apertures pr. galaxy.
N is the total number of data points.
`rms' is the root mean square standard deviation.
The uncertainty on the mean has been calculated as rms/.
The differences have been calculated as ``our''-``literature''.
Outermost aperture of R256,
which may have contamination from R269, omitted.
The expected offset between Gunn r and Kron-Cousins R of
(Jørgensen 1994) has been subtracted.
The one data value marked ``:'' in van den Bergh (1977) was omitted.

Regarding tablenote `a':
The following support the conclusion that the
magnitude for R256 for the largest aperture of
Poulain & Nieto (1994) and Burstein et al. (1987)
is significantly contaminated by signal from R269.
Their largest aperture is approximately 45'',
and the separation between R256 and R269 is approximately 101''.
At
our data
give a Johnson B surface brightness of
for R269, and
for R256.

The rms scatter of
for the Johnson U comparison with
van den Bergh (1977) is mostly due to the R269 magnitude within the
2.5'' aperture, where the difference is
.
This can be explained by the following.
The galaxy has dust in the center, which can be seen in our images
(which have good seeing).
The dust causes problems in finding the appropriate center to
use for the aperture photometry.
Even if we and van den Bergh (1977) have used the same center, a positive
magnitude difference will show up if we have better seeing than
van den Bergh.
Besides,
we had overlapping ellipses out to r=2.7'' when fitting this galaxy,
and it is not clear what the impact is on
.

From the above comparison we conclude that
the magnitude zero point is consistent with literature data within
for Gunn r,
for Johnson B, and
for Johnson U.